1. Field of the Invention
The present invention generally relates to a semiconductor acceleration sensor, and more particularly, to improvement of a semiconductor acceleration sensor for use in an air bag system of automobile, an antilock braking system (ABS), a navigation system, or other commercially available equipment and the like.
2. Description of the Prior Art
In recent years, in an air bag system of an automobile or the like, a system using an acceleration sensor has been installed as standard. An essential device part of this air bag system is an acceleration sensor for sensing a shock (acceleration). A conventional differential capacity type acceleration sensor is provided with a beam part supported by an anchor part, a movable part of a movable electrode and a mass (balance) part, and a stationary electrode part disposed to be opposed to the movable electrode, thereby detecting an acceleration according to a capacity change between the movable electrode and the stationary electrode based on a displacement of the movable part.
That is, the semiconductor acceleration sensor is composed of a sensor chip part for detecting an acceleration and a signal processing part for forming/amplifying a signal of the sensor chip. In the principle of detecting the acceleration of such an acceleration sensor, a movable electrode (mass body) positioned between a pair of stationary electrodes is displaced when subjected to an acceleration. As a result, a capacity change occurring between the stationary electrode and the movable electrode is converted into an electrical signal, thereby detecting an acceleration.
However, when in operation of the conventional acceleration sensor, there has been a problem that an electrostatic force is generated due to a potential difference between a movable electrode (mass part) and a substrate, the movable electrode or mass part adheres to the substrate, and does not function. In order to prevent the movable part from adhering to the substrate, for example, Japanese Unexamined Patent Publication No. 11-230986 discloses providing a stopper part partially protruded to the substrate and preventing the movable electrode from adhering to the substrate in all of the movable electrode, the beam part, and the lower part of the mass part.
On the other hand, in recent years, almost all of automobiles have acceleration sensors installed therein. There is a growing need for smaller and low-cost sensors. Although sensors were of metal packages at an initial stage, they are mostly replaced with those of mold resin packages. Thus, there occurs a problem that sensors made of a mold resin are easily affected by an external noise as compared with those of a metal package, and its shied effect is lowered.
In order to solve the above-described problem, the inventors of the present application made effort to improve an acceleration sensor as shown in FIGS. 1, 2A and 2B. FIG. 1 is a view showing a schematic disposition and arrangement of this improved semiconductor acceleration sensor. FIG. 2A is a top view showing a sensor chip arrangement thereof, and FIG. 2B is a sectional view taken along the line Axe2x80x94A shown in FIG. 2A.
In an assembly disposition and arrangement between the signal processing chip part and sensor chip part of the semiconductor acceleration sensor shown in FIG. 1, a die pad 2 and an external lead 6 are provided in a package exterior part 1, and a sensor chip 3 for detecting an acceleration and a signal processing chip 5 (hereinafter, referred to as xe2x80x9csignal processing ICxe2x80x9d or xe2x80x9cASICxe2x80x9d) for forming/amplifying a signal of the sensor chip are disposed on the top face of the die pad 2.
Pads 19 and 22 opposed to each other on the sensor chip 3 and the signal processing chip 5 are connected to each other via a metal wire 20, and pads disposed in a horizontal direction on the signal processing chip 5 are connected to external leads 6 via metal wires 7. In addition, on the sensor chip 3, there is provided a sealed structure in which a protective cap 4 made of an electrically conductive material such as silicon is provided in order to protect a movable part for detecting acceleration from a mold resin. In such a sensor chip 3, its electrode structure is formed in accordance with a surface micro-machining technique.
In a structure of the sensor chip 3, as shown in FIGS. 2A and 2B, an insulation oxide film 10 functioning as an insulation layer with a silicon substrate 9 is formed on the silicon substrate 9, and a first stationary electrode 11 and a second stationary electrode 12 made of a polysilicon material are disposed in plurality on the top face of the insulation oxide film 10. These first and second stationary electrodes 11 and 12 are connected to aluminum pads 19 arranged on the top face of the substrate 9 via a polysilicon wire 17. A shield electrode layer 14 to which a stationary potential (such as GND) is provided is formed to be embedded at the center of the top face part of the insulation oxide film 10, and a polysilicon wire is disposed at a periphery of the shield electrode layer 14.
In addition, a pair of beam structures 15 formed by groove-separating monocrystal silicon are formed, for example, on the top face part of the substrate 9. The beam structure 15 is cross-linked by a pair of anchor parts 16 protruding from the side of the substrate 9, is disposed at a position spaced from the top face stationary part of the substrate 9 with predetermined intervals, and is arranged so as to be movable due to beam displacement.
A main body (mass part) of the movable electrode 13 of elongated polysilicon is cross-linked between a pair of beam structures 15. The movable electrode 13 has a plurality of movable electrode protrusions 13a protruding in parallel to its side face part with equal intervals, and the first and second stationary electrodes 11 and 12 are disposed to be opposed to its side face with substantially equal intervals between the movable electrode protrusions 13a. The protective cap 4 is bonded to sealed with a cap mounting part 18 on the top face of the insulation oxide film 10 so as to cover these movable electrode part, stationary electrode part, and beam structures or the like.
In this manner, an electrode structure produced by using a micro-machining technique is very fine, and a fine capacity change is detected. Thus, this structure is easily affected by an externally applied potential (such as a noise), and sensor characteristics may be degraded. Therefore, in general, the shield electrode 14 to which the stationary potential (such as GND) is provided is disposed at the lower part of the movable electrode 13, and an effect due to an external potential (such as a noise) is eliminated, thereby suppressing such an external noise or the like.
However, in the case where the stationary potential is provided to such a shield electrode, the potential of the movable electrode is changed due to an acceleration. Thus, in the case where a potential difference with respect to the shield electrode becomes significant, there is a possibility that the movable electrode is attracted to, and is securely bonded with the shield electrode due to electrostatic attracting force, and then, becomes inoperable.
In general, the above-described protective cap 4 is usually made of an insulation material such as low melting point glass, and is pasted on a substrate. Thus, there is a problem that the potential of the protective cap is in a floating state, and a proper shield effect cannot be achieved.
The present invention has been made in order to solve the above-described problems. It is an object of the present invention to provide a small, low-cost semiconductor acceleration sensor in which the movable electrode of the semiconductor acceleration sensor is prevented from being securely bonded with the substrate with an electrostatic attracting force and from being inoperable, and in which there is solved a problem on achieving proper shield effect.
In a semiconductor acceleration sensor of the present invention, a potential identical to that of a movable electrode is given to a shield electrode, whereby disposition and arrangement for connecting a sensor chip and a signal processing IC to each other are improved so that a potential difference is eliminated between the movable electrode and the shield electrode, and an electrostatic attracting force does not occur, the secure bonding of the movable electrode is reliably prevented, and a shield effect is improved.
In order to achieve the foregoing object, according to an essential aspect of the present invention, a semiconductor acceleration sensor includes: a sensor chip connected to a signal processing IC on a die pad; a stationary electrode and a movable electrode which are arranged on a semiconductor substrate of the sensor chip; a protective cap covering the stationary electrode and movable electrode; a shield layer which is embedded in a top face part of the semiconductor substrate; and a sealing resin for sealing the stationary electrode and the movable electrode. In this configuration, at least one of the shield layer and the protective cap is electrically connected to the movable electrode, thereby being identical to each other in potential.
With the above-described arrangement, there can be provided a semiconductor acceleration sensor in which the movable electrode is prevented from being securely bonded with the substrate by an electrostatic attracting force and being inoperable, and the entire arrangement is simple and inexpensive.